Although the Arizona congresswoman was transferred last Friday to Memorial Hermann healthcare system in Houston, where she was scheduled to enter The Institute for Rehabilitation and Research (TIRR), she was admitted instead to the hospital's neurological intensive care unit.

One of the doctors involved in her care in Houston, trauma surgeon John Holcomb, MD, said that a drain had been inserted to release a buildup of fluid. Until that drain is removed or a permanent shunt is implanted, Giffords must remain in the neuro ICU.

Reid Thompson, MD, chairman of neurological surgery at Vanderbilt University in Nashville, said in an e-mail to ABC News and MedPage Today that fluid buildup is a very common problem in neurosurgery.

"In the setting of a gunshot wound, and recent surgery, it would not be unusual to build up fluid and possibly have fluid leak out -- raising the risk for an infection (meningitis)," wrote Thompson, who is not involved in Giffords' care.

The timing of the drain placement, he said, suggests that Giffords has developed a fluid leak either from inflammation in the brain or an infection.

"This creates a plumbing problem as fluid can no longer circulate out," he wrote.

According to a Houston Chronicle story, Holcomb said over the weekend that the fluid does not appear to be infected.

If the fluid buildup does not resolve within about two weeks, Thompson explained, the drain -- a potential source of infection -- would have to be replaced with a permanent shunt, which would divert spinal fluid from the ventricles of the brain to the abdomen, where it is absorbed.

"I don't see it as a setback," Thompson wrote in his e-mail. "Rather it is part of the process from her original injury. It will, however, keep her from progressing to a rehab environment quickly."

Although Giffords' transfer to the dedicated rehabilitation hospital has been delayed for an indeterminate amount of time, she will continue rehab in the ICU.

Another member of the medical team responsible for her care, neurosurgeon Dong Kim, MD, said the congresswoman "looked spectacular" when she arrived in Houston from University Medical Center in Tucson, Ariz., where she'd been cared for since being shot in the head at a public event on Jan. 8.

Kim said Giffords was alert, interactive, awake, calm, and comfortable.

He added that she had very good movement on the left side of her body and did not like it when doctors shined light in her eyes, both of which are considered good signs.

Kim noted that Giffords did not have much tone in her right arm and that over a period of about 30 minutes, she did not move it. The medical team in Tucson had reported seeing her move her right arm.

Overall, Kim said he expects Giffords to do "remarkably well," adding that the entire process, including ICU care and inpatient and outpatient rehabilitation, will probably last four to six months, regardless of how quickly she recovers.

Although her doctors are optimistic, several physicians contacted by ABC News and MedPage Today cautioned that Giffords' future function remains uncertain.

"Sadly, this is where the long-term reality of brain injury starts to hit home," Gregory O'Shanick, MD, medical director of the Center for Neurorehabilitation Services in Richmond, Va., wrote in an e-mail.

"As she increases her efforts towards becoming more independent with rehabilitation," he wrote, "the deficits will become ever more apparent and frustrating since there is no surgery, no single medication, and no artificial prosthesis to reverse the injury she sustained."

Other physicians thought it unlikely that Giffords would recover without some physical or cognitive deficits.

"I think it would be too early to ask such a question without further testing, but the reality is, most people with this type of injury usually cannot return to their previous level of functioning, especially if it was at a high level," wrote Inam Kureshi, MD, director of the head injury program at Hartford Hospital in Connecticut.

A statement from Memorial Hermann said the next update on Giffords' condition will be provided when she is transferred to the rehabilitation facility.

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